Screen printing is a printing process using a fine-mesh screen (clear widths on the order of 150 to 400 .mu.m, corresponding to a filament number of 30 to 200 per inch) and possibly a stencil which is used particularly for large-area supporting sheets of paper, plastic films or foils, metal foils, glass sheets and the like. A viscous ink or printing composition is placed on the screen and distributed over it by means of a doctor blade. In their simplest form, the machines and equipment developed for performing this process include a conventional printing table with the stencil mounted hingedly on top of the platen. A doctor blade is driven manually or electrically to squeeze the ink or screen printing composition through the screen onto a substrate. In case a great number of prints is to be provided, automatic screen printing machines are used to position the supporting substrate sheets and to squeeze the composition through the screen in a fully automatic manner. Horizontal screens are used in flat-bed printers. In cylinder screen printers, the screen is wrapped around a cylinder and is provided with the printing composition from the inside out. Modern screen printing machines turn out on the order of 2000 and more prints per hour. The screen printed sheets are dried and may be chemically modified by, for example, crosslinking the ink or printing composition.
It has been known to use the screen printing process for coating backing sheets with pressure-sensitive adhesives (PSAs). For screen printing PSAs, a variety of compositions or formulations have been used in which the adhesive comprises an organic polymer product. Because of the stringent requirements particularly in high-performance screen printing, screen printing compositions must adhere firmly to the substrate to be coated.
Japanese Laid-Open Application 62 048 780 (Toa Gosei) describes water-dispersible pressure-sensitive adhesive compositions useful for rotary screen printing which are obtained by polymerizing in an aqueous medium a monomer mixture consisting of (meth)acrylic acid esters. Polymerization in an aqueous solution calls for the use of emulsifiers or dispersing agents. The polymers have a relatively high molecular weight and are dispersable, but not soluble in water any more. These (meth)acrylic acid ester polymers must have less than 10 weight % of monomers having polar groups. The glass transition temperature of the polymers is -50.degree. C. to -30.degree. C. These PSAs are particularly suited for use in rotary screen printing on substrates such as films, textiles and paper.
Japanese Laid-Open Application 60 032 869 (Toa Gosei) proposes PSAs suited for screen printing which consist of an aqueous dispersion of (meth)acrylic acid ester copolymers. The polymers are obtained by suspension or emulsion polymerizing in water a monomer mixture comprising at least 50 weight % (meth)acrylic acid esters, less than 10 weight % of a monomer having polar groups and not more than 50 weight % of another olefinically unsaturated monomer.
Japanese KOKAI Patent No. HEI 1(1989)-297416 discloses an pressure sensitive adhesive comprised of
(A) an acryl group copolymer having a weight average molecular weight of from 50,000 to 500,000, and 1 to 10 parts by weight per 100 parts by weight of copolymer (A) of a compound having two or more ethylenically unsaturated bonding sites in one molecule and having a molecular weight of less than 1,000, and PA0 (B) an organic solvent having a boiling point range of from 100.degree. C. to 250.degree. C.
The copolymer of this KOKAI is prepared via a process in which a portion of the monomer charge (e.g., 20% by weight) is initially added to the reaction solvent after which the balance of the monomer charge is added dropwise while the entire reaction mixture is heated at 80.degree. C. This reaction is exothermic and difficult to control. Applicants have found it impossible to produce a coatable or screen-printable adhesive via the teachings of this KOKAI. The polymer either gelled and phase separated or the exotherm was uncontrollable.
Although highly thixotropic PSAs are well suited for specialized screen printing applications such as pattern printing they have no particular utility for the production of screen prints which have a large area and particularly a smooth surface, since the printed compositions do not show an adequate flow-out or running behavior. Also, since the dispersing or emulsifying agents are wetted by water, the compositions are sensitive to water and moisture will migrate through the coating to the interface and be leached out. Prints of this kind are not water-resistant and do not exhibit the required long-term stability. If exposed for extended periods to outdoor conditions, a print will lose its integrity. Also, the prior PSA products do not exhibit an acceptable balance of adhesive power, tackiness and adhesion in shear.
Attempts have been made to test screen printing PSAs uses which are composed of 100% solids so as to obviate solvent problems. A number of PSAs commercially available at present are curable by radiation-induced crosslinking. Their disadvantage is that they require the use of expensive and toxic monomers such as acryl and polyester monomers so that the residual monomers in the adhesive coating raise considerable toxicity and disposal problems. It has also been found also that these solid PSA compositions do not have the required adhesion in shear.
In DE-OS 33 46 100, a pressure-sensitive adhesive having properties of being releasable without residues is printed on a sheet material (screen printing is possible) by applying spaced cap-shaped adhesive sites having a prescribed diameter from a high solids content adhesive suspension. The adhesive is a highly concentrated aqueous dispersion on the basis of (meth)acrylic acid esters and includes very small amounts of other monomers. The cap-shaped adhesive sites are crosslinked, preferably by electron beams. In preparing the acrylic polymers, an anionic emulsifier (sodium salt of an ethoxylated sulphonated nonylphenyl) is used to make possible a polymerization of the monomers in the aqueous medium. In practice, the polymerization is performed in two stages, with a material such as 2-ethylhexylacrylate being added in time staggered amounts. The polymer dispersion is printed right away onto a random-fiber fabric or a foil using a gravure or screen printing system. The crosslinking agent is a bifunctional monomer (butanedioldiacrylate) which is incorporated in the PSA composition in the polymerization process and presumably undergoes some pre-reaction at that time already. A partial crosslinking of this kind during polymerization certainly is not undesirable as the adhesive composition of the cap-shaped sites is desired to be highly thixotropic. The molecular weight of acrylester polymers produced in an aqueous dispersion has been known to be fairly high.
The use of aqueous emulsifiers, which are present in the resultant screen print, is one of the causes of the water and moisture sensitivity of the PSA compositions of DE-OS 33 46 100. The adhesive cap is supposed to release and be removed easily. On the other hand, film-like continuous adhesive coatings are not possible with these prior pressure-sensitive adhesives.
Also available in the marketplace are adhesives based on caoutchouc or rubber which for screen printing uses are dissolved in an unpolarized aromatic solvent such as xylene. The handling of the solvents is hazardous, and they give rise to numerous disposal problems. The stability of the screen printed adhesive compositions against aging is unsatisfactory, and their handling gives rise to considerable cleaning problems. At elevated temperatures, adhesive and shear strengths are inadequate.
Copolymer PSAs based on (meth)acrylic acid and (meth)acrylic acid esters have been known for a long time. It has also been know to generate certain spectra of properties, as desired for a specific use, by judiciously selecting the composition of the pressure-sensitive adhesive. Unfortunately, many of the products offered by the adhesive industry are tailored to favor one property at the expense of all others. Copolymers of alkyl acrylate with a minor amount of acrylic acid are inherently tacky and do not require an additional tackifying resin. Further, their stability against aging is much superior to that of rubber resin-based PSAs. To this date, acrylate adhesive compositions are preferably made in an aqueous emulsion since this approach guarantees high concentrations and high molecular weights. As the first acrylate adhesives were somewhat sensitive to heat (cf. U.S. Pat. Nos. 3,617,362 and 3,707,518), it was contemplated to crosslink them (U.S. Pat. Nos. 3,740,366 and 4,077,926). Chromium(III) compounds, zinc octoate and others were used as crosslinking agents. Unfortunately, crosslinking causes some loss of tackiness; neither are the resultant adhesive and shear strength levels sufficient to justify a recommendation of these PSAs in high performance situations such as automobile components without problems.
U.S. Pat. No. 4,418,120 describes a PSA composition applied to a substrate. The PSA composition consists of isooctylacrylate or 2-ethylhexylacrylate and acrylic acid (3-7 parts by weight acrylic acid per 100 parts by weight of acrylic acid ester), a minor amount of an antioxidant and a rosin acid ester tackifying resin. The adhesive shows good adhesion to stainless steel, acrylonitrile/butadiene/styrene terpolymer (AsS), low-density polyethylene (LDPE) and isotactic polypropylene. The adhesive strength in shear is still very good at 70.degree. C. For the production of this PSA composition, a solvent mixture of the copolymer is prepared in a non-polar, highly volatile organic solvent, and the mixture is adjusted to an inherent viscosity of 0.75 to 1.5 dl/g. In practice, the viscosity is adjusted by adding solvents such as toluene to a much more viscous composition. The antioxidant is an inhibited phenol, the tackifier a rosin ester derivative. Prior to its application to a substrate, an organic solution of N,N'-bis-1,2-propyleneisophthalamide (also referred to as a bisamide) is added to the adhesive solution, and after its application to a substrate the coated structure is heated, whereby the solvent is removed and the acrylate adhesive is crosslinked. The acrylate adhesives are copolymers which may contain other monomers, such as itaconic acid, methacrylic acid, (meth)acrylamide, vinyl acetate and n-butyl-acrylate, only in minor amounts.
Nothing is reported regarding the screen printing of PSA products of this nature. Their use is likely to be problematic as the adhesive copolymer cannot be adjusted for elevated concentrations, the handling of toluene or benzene solutions in the printing phase is hazardous, and the removal and disposal of the solvent will give rise to considerable problems. The aforesaid prior patent is silent as to a maximation of the water and moisture stability of the products.
U.S Pat. No. 4,737,559 describes medical adhesive compositions suited for skin applications which comprises a copolymer of an acrylate or methacrylate ester of a non-tertiary alcohol having an average of 4 to 12 carbon atoms and a monoethylenically unsaturated aromatic ketone monomer (particularly p-acryloxybenzophenone). The copolymer may be polymerized by ultraviolet (UV) radiation, which also results in an adequate creep compliance. The initial adhesion to the skin is highly favorable and does not increase with time. It is possible to include with the acryl monomers another comonomer such as (meth)acrylic acid, itaconic acid, (meth)acrylamide, acrylonitrile, methacrylnitrile, vinyl acetate and N-vinylpyrrolidone. This patent is silent as to solvents, so that no conclusions can be drawn as to any suitability for screen-printing applications. Because of their extremely high molecular weight and the solvents taught as useful in the preparation, the adhesives disclosed in the patent are unlikely to be screen printable; rather, and as shown below, they are likely to have a tendency to "string".